Attention is directed to a masters thesis by S. N. Mishra entitled "Real Time Ink Correction Module" (MIT, May, 1981) and a masters thesis by E. A. Lee entitled "Digital Color Translation" (MIT, May, 1981) both prepared under my supervision and hereby incorporated by reference. Attention is also directed to article written by myself and others, D. E. Troxel et al. entitled "Automated Engraving of Gravure Cylinders" IEEE Transactions on Systems, Man and Cybernetics, Vol. SMC-11, No. 9, pp 585-596 (September 1981) also incorporated herein by reference.
This invention relates to color processing and, in particular, to computer-assisted color editing systems. Its primary use is in color printing by the three most common processes--offset lithography, letterpress, and gravure. It is also applicable to plateless printing such as Xerography, to photography, and to any other color reproduction process which uses a small number of colorants, usually three or four, in various mixtures, more or less to match the colors of the original. In all these processes, it is usual to form three separate images by photographing or scanning the original through three different color filters, normally red, green, and blue. These three color separations are combined in various ways to produce printing plates or the equivalent thereof, which in turn control the amount of each colorant deposited at each point of the reproduction. A problem common to these processes is that the exact combination of colorants required for the match is not related, in any simple way, to measurements which can be made on the original. Achieving high quality reproduction is therefore difficult, costly, time-consuming, and requires great skill and experience on the part of the operator.
Another problem in these systems is that in almost all cases, exact matching of the original is neither possible nor desirable. Originals may not be perfect and, of at least equal importance, the gamut achievable with a particular set of inks and paper is often less than that of the original. Therefore deliberate distortion must be introduced. While this is possible, it is quite difficult to predict the precise effect of such distortion, so that in many cases trial plates must be made and proofs printed, thus adding to the cost.
In order better to describe my invention and its relation to the prior art, the following terms are defined at the outset:
Tristimulus Values--The amounts of three primary colored lights, which, when added, produce a visual, or "colorimetric" match with an original color. Such a set of primaries consists of the red, green, and blue phosphor colors of a TV tube, in which case the tristimulus values are called R, G, and B.
Appearance Signals--Values produced by any reversible transformation of RGB. Luminance/chrominance (LC1C2) and luminance, hue, and saturation (LHS) are two common sets.
Color--The specification of a colored stimulus requiring at least three component values.
Luminance--That aspect of a colored stimulus relating to its intensity.
Hue--That aspect of a colored stimulus relating to its color name.
Saturation--That aspect of a colored stimulus relating to its purity, or absence of contamination with white.
Chrominance--That aspect of a colored stimulus relating to its hue and saturation. The saturation is aproximately the ratio of chrominance amplitude to luminance.
Color Space--A three-dimensional space in which each point corresponds to a color, including both luminance and chrominance aspects. RGB form such a space. LHS form a set of cylindrical coordinates in color space. The L-axis is the diagonal of RGB space, so that L=0 where R=G=B=0, and L=max where R,G, and B are max. The C1C2 plane is perpendicular to the L-axis in LC1C2 space. The hue (angle) and chrominance (amplitude) are polar coordinates in the C1-C2 plane.
Lightness--A non-linear transformation of luminance in which equal increments are equally perceptible.
Density--The negative logarithm, to the base ten, of the reflectance or transmittance of a point in an image. In the case of colored inks or dyes, the density is measured through an appropriate color filter. The density is approximately proportional to the quantity of ink laid down. CMYK refer to the densities of cyan, magenta, yellow, and black ink normally used in printing.
Gamut--The range of colors producible with a set of inks, lights, or other colorants. The gamut can conveniently be described in terms of a particular region of a color space.
Transparent--That property of an optical medium such as a dye or an ink in which each ray of incident light is transmitted without change of direction, but attenuated (multiplied) by a factor which is always unity or less.
Standard Translation--When the reproduction gamut is smaller than the gamut of the original, the usual case, the dynamic range (contrast range) of the original must be compressed and in most cases, some highly saturated colors must be desaturated. Some special colors, such as skin tones, if they cannot be accurately reproduced, are preferably distorted in certain ways. All these changes, taken together, constitute the standard translation.
Color Mixture Curves (CMC's)--The spectral transmission curves for a set of color separation filters which produce signals which are tristimulus values with respect to a certain set of primaries.
Additive Mixture--The type of color mixture in which the light of each component is summed. A color TV tube has this type of mixture, which obeys particularly simple mixture rules.
Subtractive Mixture--The type of color mixture in which the spectral transmittance curves of the components multiply. Color films behave this way, approximately. The mixture rules are more complicated, but the resultant color can be accurately predicted. Ink mixtures as encountered in typical printing processes are more nearly subtractive than additive, but are extremely difficult to predict accurately because of non-ideal behavior of the inks.
Tone Scale Memory--A table implemented in digital hardware or in software which serves the purpose of a non-linear transformation. The addresses, typically 256, are the various levels of the input signal, while the contents, typically 8 bits at each location, are the corresponding levels of the output signal.
Colorimeter--An instrument or method for measuring the tristimulus values of arbitrary color samples.
Various patents and publications have disclosed computer-assisted color processing systems. An article by W. L. Rhodes entitled "Proposal for an Empirical Approach to Color Reproduction" in Color Research and Application Vol. 3, No. 4, pp 197-201 (winter 1978) summaries the history of color correction systems. An article by P. Pugsley entitled "Pre-Press Picture Processing in the Graphic Arts Industry" in IEEE Transactions on Communications, Vol. COM-29, No. 12, pp 1891-1897 (December 1981) described the equipment and techniques used in preparing pages containing color images for printing including the use of a look-up table technique for determining required ink densities. This look-up table system is also described in U.S. Pat. No. 3,893,166 issued to Pugsley on July 1, 1975.
A number of other patents also describe look-up table systems including U.S. Pat. No. 3,612,753 issued to Korman on Oct. 12, 1971; U.S. Pat. No. 4,058,828 issued to Ladd on Nov. 15, 1977; U.S. Pat. No. 4,060,829 issued to Sakamoto on Nov. 29, 1977; and U.S. Pat. No. 4,127,871 issued to Sakamoto on Nov. 28, 1978.
Color editing systems for printing are suggested but not fully described in U.S. Pat. No. 4,037,249 issued to Pugsley on July 19, 1977 and in Japanese Specification No. 55-115043 by Ahei and Tamada published Sept. 4, 1980. There exists a need for a generally all-encompassing, color reproduction system, particularly for rotogravure processes and the like. The system should be capable of producing color images employing the entire gamut of colors physically achievable with all possible combinations of the colorants. Moreover, the system should provide precise compensation for the effects of ink and paper which are made automatically without depending on operator skill while maintaining the capability for operator-initiated aesthetic alterations.